Photodectector Having Dark Current Correction

ABSTRACT

A photodetector and method for making the same is disclosed. The photodetector includes a substrate having first, second, and third photodiodes and first and second pigment filter layers. The first, second, and third photodiodes generate first, second, and third photodiode output signals, respectively, each photodiode output signal being indicative of a light intensity incident on that photodiode and a dark current that is independent of the light intensity. The first and second pigment filter layers overlie the first and second photodiodes while a layer having both the first and second pigment filter layers overlie the third photodiode. An output circuit combines the first and third photodiode output signals to provide a first corrected output signal and combines the second and third photodiode output signals to provide a second corrected output signal.

BACKGROUND OF THE INVENTION

Inexpensive photodetectors that measure the intensity of light in anumber of wavelength bands are required in a number of devices. Forexample, light sources that utilize red, blue, and green LEDs togenerate light that is perceived as being a particular color oftenutilize photodetectors in a servo loop that maintains the output of theLEDs at predetermined levels to compensate for aging. The photodetectorsare used to measure the output of each LED. A controller adjusts theaverage current to each LED such that the measured outputs aremaintained at target values determined by the perceived color of lightthat is to be generated.

In one commonly used type of photodetector, the photodetector utilizesphotodiodes that are covered by pigment filters that limit the responseof each of the photodiodes to light in a corresponding band ofwavelengths. The signals from the various photodiodes are processed toprovide signals that represent the output of each of the LEDs. Thesignal from each photodiode is determined by the incident light, thebandpass filter characteristics of the pigment and various backgroundsignals that are present independent of the intensity level of the lightreaching the photodiode. The light-independent signals are oftenreferred to as the “dark current”. The errors generated by the darkcurrent can be significant in a number of applications; hence, schemesfor correcting for the dark current have been developed. In addition,removing the contributions to the final signals that result from thedark current improves the dynamic range of the photodetector, and hence,the photodetector can be used to control the LEDs over a larger range oflight intensities.

In one dark current correction scheme, the errors generated by the darkcurrent are removed by measuring the output of the photodiode when nolight is present and then subtracting the measured signal value from thesignals generated by the photodiode in the presence of light. In thisarrangement, the photodiodes are identical in structure and differ onlyin the type of pigment filter that overlies each photodiode. Anadditional photodiode that is covered by an opaque layer that blocks alllight is included in the photodetector. The signal from this photodiodeis then subtracted from that generated by the photodiodes that arecovered with the various pigment filters. This scheme, however,significantly increases the cost of the photodetectors, since additionalmasking steps are needed to provide the opaque layer over the additionalphotodiode.

SUMMARY OF THE INVENTION

The present invention includes a substrate having first, second, andthird photodiodes and first and second pigment filter layers. The first,second, and third photodiodes generate first, second, and thirdphotodiode output signals, respectively, each photodiode output signalbeing indicative of a light intensity incident on that photodiode and adark current that is independent of the light intensity. The firstpigment filter layer overlies the first photodiode but not the secondphotodiode, and is transparent to light in a first band of wavelengthsand opaque to light in a second band of wavelengths. The second pigmentfilter layer overlies the second photodiode, but not the firstphotodiode. The second pigment filter layer is transparent to light inthe second band of wavelengths and opaque to light in the first band ofwavelengths. A layer that includes the first and second pigment filterlayers overlies the third photodiode. An output circuit combines thefirst and third photodiode output signals to provide a first correctedoutput signal and the second and third photodiode output signals toprovide a second corrected output signal. In one aspect of theinvention, the first, second, and third photodiodes have the same darkcurrent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a prior art photodetector thatutilizes pigment filters.

FIG. 2 is a cross-sectional view of a photodetector according to oneembodiment of the present invention.

FIG. 3 is a schematic drawing of a photodiode according to anotherembodiment of the present invention.

FIG. 4 is a schematic drawing of a photodiode according to anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The manner in which the present invention provides its advantages can bemore easily understood with reference to FIG. 1, which is across-sectional view of a prior art photodetector that utilizes pigmentfilters. Photodetector 20 is typically constructed from a die 21 having4 photodiodes fabricated thereon. Photodiodes 22-24 are used to measurethe intensity of light in three wavelength bands that are determined bypigment filters 25-27, respectively. Photodiode 28 is covered by anopaque layer 29 and is utilized to measure the dark current. The pigmentfilters are applied by photolithographic steps that require a number ofmasking and deposition steps. A similar photolithographic step is neededfor applying layer 29.

Refer now to FIG. 2, which is a cross-sectional view of a photodetectoraccording to one embodiment of the present invention. In photodiode 30,the opaque layer that covered dark current detector 28 is replaced by astack 31 of pigment filters that utilize the same pigments as filters25-27. The signal from photodiode 28 provides the dark current signalthat was previously measured using opaque layer 29. Hence, bysubtracting the signal from photodiode 28 from the signals generated byeach of the other photodiodes, the photodiode signals can be correctedfor dark current.

Refer now to FIG. 3, which is a schematic drawing of a photodiodeaccording to another embodiment of the present invention. Photodiode 40utilizes the chip shown in FIG. 2 to generate 3 corrected photodiodesignals 47-49. The corrected signals are generated by subtracting theoutput of photodiode 28 from each of the outputs of photodiodes 22-24using subtraction circuits 42-44.

The above-described embodiments of the present invention assume that theincident light is restricted to light in the visual portion of theoptical spectrum. In particular, the above-described embodiments assumethat the incident light is devoid of light in the infrared portion ofthe spectrum. Most commonly used photodiodes are sensitive to light inthe infrared portion of the spectrum. In addition, the pigment filtersthat are typically used, transmit light in the infrared portion of thespectrum in addition to the light in the band of wavelengths ofinterest.

If each of the pigment filters is essentially transparent in theinfrared region of the spectrum, the arrangement shown in FIG. 3 willalso correct for any infrared radiation in the incident light. In thiscase, the signal from each of the photodetectors will also include acomponent that has a magnitude equal to that of the infrared radiation.Since all of the photodiodes are assumed to be identical, thesubtraction of the signal from the photodiode having the stacked pigmentfilters will also correct the signals for the infrared component in theincident light. If pigment filters partially attenuate the infraredlight, then the correction provided by the embodiment shown in FIG. 3will only partially correct for the infrared component in the incidentlight. However, the correction will still be better than that providedby utilizing an opaque layer over the photodiode used to measure thedark current.

It should be noted that the dark current correcting filter utilized inthe present invention does not require any new fabrication steps beyondthose used to deposit the other bandpass pigment filters. For example,in a lithographic deposition scheme, each photodiode is provided with apigment filter by masking the other photodiodes and then depositing thepigment over the photodiode in question. The mask is then removed and anew mask that covers all but the next photodiode is introduced. A layerof pigment is then deposited over the unmasked layers. The process isrepeated until each photodiode is covered with the pigment correspondingto that photodiode. The dark current filter of the present invention canbe constructed at the same time as the bandpass filters by leaving thearea over the photodiode in question unmasked during each of thedeposition steps used to deposit the other pigment layers. Hence, no newmasks and deposition steps need be used.

In one embodiment of the present invention, the pigments are the red,blue, and green pigments that are normally utilized to constructphotodetectors. Such filters are used in imaging arrays that are used indigital cameras, and hence, will not be discussed in detail here.

The above-described embodiments of the present invention utilize apigment stack having all of the layers that are applied to the otherphotodiodes to construct the opaque layer that is used to shield thephotodiode used to generate the dark current signal. However, less thanthe entire set of pigment filters could be utilized. For example, thecombination of a blue and red bandpass filter could, in some cases, besufficiently opaque to provide the required shielding of the underlyingphotodetector. This more limited stack of filters is useful in cases inwhich the thicker full pigment filter stack cannot be utilized due toproblems maintaining the thicker stack. For example, the thicker stackof filters could be subject to detachment during temperature cycling.

It should be noted that such a limited stack of filter layers can stillbe provided without any new masking and/or deposition steps. Forexample, consider the case in which the green pigment filter is to beomitted from the filter stack. The deposition of the green pigmentincludes three steps. The first step deposits a patterned mask over theareas that are not to be covered by the green pigment in the finalphotodetector. In the second step, the pigment is deposited, andfinally, the mask is removed in the third step. If the green pigment isto be eliminated from the stack, the patterned mask is set such that themask extends over the area above the dark current photodiode.

The above-described embodiments of the present invention utilizephotodiodes that are identical in structure for both the dark currentsensing photodiode and the band pass sensing photodiodes. However,embodiments in which the photodiodes have different dimensions couldalso be utilized. For example, the photodiodes could have areas that areinteger multiples of the area of one of the photodiodes. In such cases,the ratio of the dark current in the dark current sensing photodiode tothe dark current in the band pass sensing photodiodes would need to beknown. The output from the dark current sensing photodiode could then bescaled with an appropriate amplifier or attenuator to account for thedifferent structures. Refer now to FIG. 4, which is a schematic drawingof a photodiode according to another embodiment of the presentinvention. Photodiode 50 differs from photodiode 40 discussed above inthat photodiode 28 is replaced by photodiode 58 that has a differentstructure than the remaining photodiodes. In this case, photodiode 58has a dark current that is different from the dark currents of theremaining photodiodes; however, the dark current of photodiode 58 has afixed value, and hence, the dark current from photodiode 58 can still beutilized to correct for the dark current of the other photodiodes byutilizing an appropriate amplifier or attenuator to scale the output ofphotodiode 58 before subtracting that output from the output of each ofthe other photodiodes. This scaling operation is performed by amplifiers51-53. It is to be understood that amplifiers 51-53 could have gainsthat are less than 1, i.e., amplifiers 51-53 could be attenuators.

The above discussion refers to the pigment layers as being “transparent”and “opaque” to light in various bands of wavelengths. Ideally, atransparent layer has 100% transmission for light in the relevantwavelength band and the opaque layer has 0% transmission for light inthe relevant wavelength band. However, it will be appreciated thatpigments that are only partially transparent or partially opaque couldbe utilized and still provide a photodetector that is superior to theprior art photodetectors in particular applications. Hence, the term“transparent layer” is defined to include layers that have sufficienttransmission to allow the photodiode under the layer to measure light inthe relevant wavelength band in the presence of light in a band in whichthe layer is said to be opaque even though the layer has sometransmission in the band to which it is said to be opaque. Inparticular, the term “transparent layer” includes layers that havetransmissions greater than 60 percent. Similarly, the term “opaquelayer” is defined to include layers that have transmissions less than 30percent.

Various modifications to the present invention will become apparent tothose skilled in the art from the foregoing description and accompanyingdrawings. Accordingly, the present invention is to be limited solely bythe scope of the following claims.

1. A photodetector comprising: a substrate having first, second, andthird photodiodes that generate first, second, and third photodiodeoutput signals, respectively, each photodiode output signal beingindicative of a light intensity incident on that photodiode; a firstpigment filter layer overlying said first photodiode but not said secondphotodiode, said first pigment filter layer being transparent to lightin a first band of wavelengths and opaque to light in a second band ofwavelengths; a second pigment filter layer overlying said secondphotodiode, but not said first photodiode, said second pigment filterlayer being transparent to light in said second band of wavelengths andopaque to light in said first band of wavelengths; a layer comprisingsaid first and second pigment filter layers overlying said thirdphotodiode; and an output circuit that combines said first and thirdphotodiode output signals to provide a first corrected output signal andthat combines said second and third photodiode output signals to providea second corrected output signal.
 2. The photodetector of claim 1wherein said output circuit subtracts a signal that is a multiple ofsaid third photodiode output signal from said first and secondphotodiode output signals.
 3. The photodetector of claim 1 wherein saidfirst, second, and third photodiodes have areas that are integermultiples of the area of one of said photodiodes.
 4. The photodetectorof claim 1 further comprising a fourth photodiode and a third pigmentfilter layer overlying said fourth photodiode but not said first andsecond photodiodes, said fourth photodiode generating a fourthphotodiode output signal indicative of a light intensity incident onsaid fourth photodiode and a dark current that is independent of thatlight intensity, wherein said output circuit further combines said thirdand fourth photodiode output signals to provide a third corrected outputsignal.
 5. The photodetector of claim 4 wherein said third correctedoutput signal is corrected for said dark current.
 6. The photodetectorof claim 4 wherein said third pigment layer also overlies said thirdphotodiode.
 7. The photodetector of claim 1 wherein each of saidphotodiodes generates a dark current in the absence of light beingreceived by said photodiodes and wherein said first and second outputsignals depend less on said dark current than said first and secondphotodiode output signals.
 8. A method for determining the lightintensity of a light signal in each of a plurality of spectral bands,said method comprising: providing first, second, and third photodiodesthat generate first, second, and third photodiode output signals,respectively, each photodiode output signal being indicative of a lightintensity incident on that photodiode; providing a first pigment filterlayer overlying said first photodiode but not said second photodiode,said first pigment filter layer being transparent to light in a firstband of wavelengths and opaque to light in a second band of wavelengths;providing a second pigment filter layer overlying said secondphotodiode, but not said first photodiode, said second pigment filterlayer being transparent to light in said second band of wavelengths andopaque to light in said first band of wavelengths; providing a layercomprising said first and second pigment filter layers overlying saidthird photodiode; and combining said first and third photodiode outputsignals to provide an estimate of said light intensity in said firstband of wavelengths and combining said second and third photodiodeoutput signals to provide an estimate of said light intensity in saidsecond band of wavelengths.
 9. The method of claim 8 wherein said first,second, and third photodiodes have areas that are integer multiples ofthe area of one of said photodiodes.
 10. The method of claim 8 furthercomprising providing a fourth photodiode and a third pigment filterlayer overlying said fourth photodiode but not said first and secondphotodiodes, said fourth photodiode generating a fourth photodiodeoutput signal indicative of a light intensity incident on said fourthphotodiode and a dark current that is independent of that lightintensity, said third pigment filter being transparent in a third bandof wavelengths, wherein said method further combines said third andfourth photodiode output signals to provide an estimate of a lightintensity in said third band of wavelengths.
 11. The method of claim 10wherein said estimate of said light intensity in said third band ofwavelengths is corrected for said dark current.
 12. The method of claim10 wherein said third pigment layer also overlies said third photodiode.13. The method of claim 8 wherein each of said photodiodes generates adark current in the absence of light being received by said photodiodesand wherein said estimates of said light intensities in said first andsecond bands of wavelengths depend less on said dark current than saidfirst and second photodiode output signals.
 14. A method for fabricatinga photodetector, said method comprising: providing a substrate havingfirst, second, and third photodiodes that generate first, second, andthird photodiode output signals, respectively, each photodiode outputsignal being indicative of a light intensity incident on thatphotodiode; depositing a first pigment filter layer overlying said firstphotodiode and said third photodiode but not said second photodiode,said first pigment filter layer being transparent to light in a firstband of wavelengths and opaque to light in a second band of wavelengths;and depositing a second pigment filter layer overlying said secondphotodiode and said third photodiode, but not said first photodiode,said second pigment filter layer being transparent to light in saidsecond band of wavelengths and opaque to light in said first band ofwavelengths.
 15. The method of claim 14 wherein said substrate furthercomprises an output circuit that combines said first and thirdphotodiode output signals to provide a first corrected output signal andthat combines said second and third photodiode output signals to providea second corrected output signal.
 16. The method of claim 14 whereinsaid first, second, and third photodiodes have areas that are integermultiples of the area of one of said photodiodes.
 17. The method ofclaim 15 wherein said substrate further comprises a fourth photodiodeand a third pigment filter layer overlying said fourth photodiode butnot said first and second photodiodes, said fourth photodiode generatinga fourth photodiode output signal indicative of a light intensityincident on said fourth photodiode and a dark current that isindependent of that light intensity, wherein said output circuit furthercombines said third and fourth photodiode output signals to provide athird corrected output signal.
 18. The method of claim 17 wherein saidthird corrected output signal is corrected for said dark current. 19.The method of claim 17 wherein said third pigment layer also overliessaid third photodiode.
 20. The method of claim 14 wherein each of saidphotodiodes generates a dark current in the absence of light beingreceived by said photodiodes and wherein said first and second outputsignals depend less on said dark current than said first and secondphotodiode output signals.